Three people remain unaccounted for and 15 have been rescued after a helicopter ditched in the sea off Shetland.

Lifeboats from Aith and Lerwick have been sent to the scene of the reported crash, two miles west of Sumburgh airport on Shetland.

A coastguard rescue helicopter that was sent to the scene returned to Lerwick with nine passengers, according to the BBC. Eight people walked off the aircraft and one person was carried off on a stretcher.

A coastguard spokesman said they received a distress call at about 7pm. The helicopter's life rafts were found empty and some wreckage from the aircraft has started to wash up at the southern end of Sumburgh.

"There were 18 people on board and 15 have been recovered, there is still an ongoing search and rescue mission for the three missing people," a spokeswoman said. The 15 people were taken to Gilbert Bain hospital in Lerwick.

"The people that were involved are in varying stages of injury, no one has walked away from this without a scratch."

The helicopter was on approach to Sumburgh Airport when it went down. The airport has been closed so that emergency services can deal with the incident.

The Super Puma helicopter was operated by helicopter services company CHC and was taking people to and from oil and gas platforms in the North Sea.

"We can confirm that an L2 aircraft has landed in the water, approximately two miles west of Sumburgh," a statement from the company said.

"The aircraft was on approach to Sumburgh Airport at approximately 6.20pm when contact was lost with air traffic control.

"We can confirm there were 16 passengers on board, and two crew."

Investigators from the Air Accidents Investigation Branch were travelling to the scene this evening.

A spokesman for the Department for Transport said: "The AAIB is aware of the incident and has deployed a team".

A Northlink ferry which was travelling between Shetland and Aberdeen has been diverted to the scene.

Last year, two helicopters ditched in the North Sea only six months apart. All passengers and crew were rescued in both incidents, which were found to be caused by gearbox problems.

In October, 17 passengers and two crew were rescued from life rafts by a passing vessel after the helicopter, which was carrying an oil crew from Aberdeen to a rig 86 miles north west of Shetland, was forced to ditch.

Previously, in May 2012 all 14 passengers and crew members on a Super Puma helicopter were rescued after it ditched about 30 miles off the coast of Aberdeen. The helicopter was on a scheduled flight from Aberdeen airport to a platform in the North Sea at the time.